This invention relates to an interferometric sonar which utilizes the non-linear properties of a sea-water environment. In underwater acoustics, the search for objects such as mines on the sea floor calls for the use of a high-resolution sonar in order to identify such objects. Moreover, searching for mines is advantageously carried out at frequencies of a few tens of kHz (designated as low frequencies) in order to permit detection and location of buried mines. Acoustic energy in fact penetrates to a significant extent into sands and sediments only at frequencies which do not exceed 30 kHz. Furthermore, mines may be covered with so-called anechoic materials for absorbing sound waves but these materials are very effective only at high frequencies.
The use of low frequencies with good resolution calls for sonar transducer arrays of substantial size or synthetic antennas sonar transducer arrays involving complex technology.
As disclosed in particular in U.S. Pat. No. 4,234,939 (incorporated herein by reference), it is known to construct a high-resolution active sonar having a receiving transducer array comprising an array or hydrophones and a sending transducer comprising two transducers placed at the ends of the receiving transducer array. In order to ensure good coverage of an angular sector, transmission by the two end transducers takes place alternately in phase or in phase opposition. It is also a known practice to transmit at two high frequencies and to receive at the difference frequency as a result of non-linearities of the propagation medium.